Due to miniaturization, sensor arrays usually suffer from leakage currents that degrade an image captured by the sensor arrays. The use of integrated microelectronic elements (like transistors or photodiodes) in monolithic sensors is an important technique that allows the fabrication of very small radiation sensor pixels, but due to the type of materials and the very small dimensions and distances between electronic elements, leakage currents often occur. One of the characteristics of the sensing element contributing to this effect is its limited sensitivity. When impinging radiation surpasses a predetermined threshold limit, the sensor saturates, and further increase of radiation intensity may lead to leakage. Some solutions have been proposed. For instance, use of pixels with a wide area may lead to an increase of saturation limit. This solution is very limited, due to the fact that a sensor array should be able to have high sensor density (e.g. high number of pixel per inch) in order to be competitive.
Other solutions remove excess charge by draining, thus avoiding leakage, in saturated pixels. While leakage and related effects (as ghosting or blooming) are reduced, the sensor saturates and the image quality is not optimal.
Dynamic range can be defined as the ratio between the smallest and the highest radiation intensity capable of producing a variation in the radiation sensor. It is desirable to have high dynamic range (HDR) in a sensor, while at the same time reducing leakage currents in a sensor array, in order to obtain an optimal configuration suitable for sensor arrays. Several alternatives and sensor configurations have been proposed.
US2012/0193516 discloses a pinned photodiode (PPD) with capacitors for storing charge, increasing dynamic range. The capacitance of the capacitors is higher than the capacitance of the sensor. Additionally, a lateral overflow barrier can be used to reduce leakage current during charge generation and collection in the photodiode.
The fabrication of a sensor suitable for sensor arrays, allowing a reduced leakage and high dynamic range, is desirable, while simultaneously avoiding increasing the area of the sensor (thus avoiding a reduction of resolution), avoiding reducing the sensitive area (thus obtaining a good saturation) and avoiding costly manufacturing routes.